U.S. patent number 10,333,313 [Application Number 15/748,717] was granted by the patent office on 2019-06-25 for electrical assembly.
The grantee listed for this patent is General Electric Technology GmbH. Invention is credited to Andrzej Adamczyk, Carl David Barker, Robert Stephen Whitehouse.
United States Patent |
10,333,313 |
Whitehouse , et al. |
June 25, 2019 |
Electrical assembly
Abstract
There is provided an electrical assembly for use in an
electrical system. The electrical assembly comprises a DC path. The
DC path includes: a DC power transmission medium; and a current
commutation device, the current commutation device including a
switching element and an energy absorbing element, the switching
element arranged to permit a current flowing, in use, through the
DC path to flow through the switching element and at the same time
bypass the energy absorbing element, wherein the electrical
assembly further includes a control unit programmed to selectively
control the switching of the switching element to commutate the
current directly from the switching element to the energy absorbing
element in order to increase the resultant voltage drop caused by
the flow of direct current through the DC path in which the current
commutation device is connected and thereby oppose the flow of the
current through the DC path.
Inventors: |
Whitehouse; Robert Stephen
(Stafford, GB), Barker; Carl David (Stafford,
GB), Adamczyk; Andrzej (Stafford, GB) |
Applicant: |
Name |
City |
State |
Country |
Type |
General Electric Technology GmbH |
Baden |
N/A |
CH |
|
|
Family
ID: |
54062875 |
Appl.
No.: |
15/748,717 |
Filed: |
July 28, 2016 |
PCT
Filed: |
July 28, 2016 |
PCT No.: |
PCT/EP2016/068111 |
371(c)(1),(2),(4) Date: |
January 30, 2018 |
PCT
Pub. No.: |
WO2017/017240 |
PCT
Pub. Date: |
February 02, 2017 |
Prior Publication Data
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|
|
Document
Identifier |
Publication Date |
|
US 20190013674 A1 |
Jan 10, 2019 |
|
Foreign Application Priority Data
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|
|
|
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Jul 30, 2015 [GB] |
|
|
1513402.6 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H02J
3/36 (20130101); H01H 9/548 (20130101); H02J
1/08 (20130101); H01H 33/596 (20130101); H02J
2003/365 (20130101); Y02E 60/60 (20130101) |
Current International
Class: |
H02M
5/458 (20060101); H01H 33/59 (20060101); H02J
1/08 (20060101); H01H 9/54 (20060101); H02J
3/36 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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104620345 |
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May 2015 |
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CN |
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3 032 677 |
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Jun 2016 |
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EP |
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2 670 013 |
|
Sep 2016 |
|
EP |
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2519791 |
|
May 2015 |
|
GB |
|
2540813 |
|
Feb 2017 |
|
GB |
|
2012/055447 |
|
May 2012 |
|
WO |
|
2013/127463 |
|
Sep 2013 |
|
WO |
|
2014/177874 |
|
Nov 2014 |
|
WO |
|
2015/078525 |
|
Jun 2015 |
|
WO |
|
Other References
Combined Search and Examination Report issued in connection with
corresponding GB Application No. 1513402.6 dated Jan. 21, 2016.
cited by applicant .
International Search Report and Written Opinion issued in
connection with corresponding PCT Application No. PCT/EP2016/068111
dated Oct. 13, 2016. cited by applicant .
International Preliminary Report on Patentability issued in
connection with corresponding PCT Application No. PCT/EP2016/068111
dated Jan. 30, 2018. cited by applicant .
First Office Action and Search Report issued in connection with
corresponding CN Application No. 201680044379.3 dated Nov. 12, 2018
(English Translation not Available). cited by applicant.
|
Primary Examiner: Zhang; Jue
Assistant Examiner: Demisse; Afework S
Attorney, Agent or Firm: Eversheds Sutherland (US) LLP
Claims
The invention claimed is:
1. An electrical assembly, the electrical assembly comprising a DC
path, the DC path including: a DC power transmission medium; and a
current commutation device, the current commutation device
including a switching element and an energy absorbing element, the
switching element arranged to permit a current flowing, in use,
through the DC path to flow through the switching element and at
the same time bypass the energy absorbing element, wherein the
electrical assembly further includes a control unit programmed to
selectively control the switching of the switching element to
commutate the current directly from the switching element to the
energy absorbing element in order to increase the resultant voltage
drop caused by the flow of direct current through the DC path in
which the current commutation device is connected and thereby
oppose the flow of the current through the DC path.
2. The electrical assembly according to claim 1; wherein the
current commutation device is configured to have a voltage rating
that enables the energy absorbing element to increase the resultant
voltage drop caused by the flow of direct current through the DC
path in which the current commutation device is connected and
thereby oppose the flow of the current through the DC path.
3. The electrical assembly according to claim 1, wherein the DC
path further includes: a mechanical switch connected to the DC
power transmission medium; and a controller configured to
selectively operate the mechanical switch to switch the DC power
transmission medium out of circuit, wherein the control unit is
programmed to selectively control the switching of the switching
element to commutate the current directly from the switching
element to the energy absorbing element to thereby force the
current in the DC power transmission medium to drop to a value that
permits safe opening of the mechanical switch prior to the
controller operating the mechanical switch to switch the DC power
transmission medium out of circuit.
4. The electrical assembly according to claim 3, wherein the
control unit is programmed to selectively control the switching of
the switching element to commutate the current directly from the
switching element to the energy absorbing element to thereby force
the current in the DC power transmission medium to drop to a value
that permits arcless opening of the mechanical switch prior to the
controller operating the mechanical switch to switch the DC power
transmission medium out of circuit.
5. The electrical assembly according to claim 3, wherein the
control unit is programmed to selectively control the switching of
the switching element to commutate the current directly from the
switching element to the energy absorbing element to thereby damp
any power oscillation present in the DC path prior to the
controller operating the mechanical switch to switch the DC power
transmission medium out of circuit.
6. The electrical assembly according to claim 3, wherein the DC
path includes a DC switchgear, the mechanical switch forming part
of the DC switchgear, and/or wherein the mechanical switch is a
disconnecter.
7. The electrical assembly according to claim 3, wherein the
current commutation device is connected in series with the
mechanical switch.
8. The electrical assembly according to claim 1, wherein the
switching element is an electronic switching element and/or a
semiconductor switching element.
9. The electrical assembly according to claim 1, wherein the energy
absorbing element includes a linear resistor and/or a non-linear
resistor, and/or wherein the energy absorbing element is connected
in parallel with the switching element.
10. The electrical assembly according to claim 1, wherein the
current commutation device is connected in series with the DC power
transmission medium.
11. The electrical assembly according to claim 1, wherein the DC
path further includes an additional DC power transmission medium
operably connected to the DC power transmission medium.
12. The electrical assembly according to claim 11, wherein the
current commutation device is connected in series with the
additional DC power transmission medium, or wherein the DC path
further includes an additional current commutation device, the
current commutation device and the additional current commutation
devices being connected in series with the DC power transmission
medium and the additional DC power transmission medium
respectively.
13. The electrical assembly according to Claire wherein the control
unit is programmed to selectively switch the switching element on
and off a plurality of times to control the commutation of the
current directly from the switching element to the energy absorbing
element.
14. The electrical assembly according to claim 1, wherein the DC
path further includes a current bypass device arranged to permit
selective formation of a current bypass path, and the current
bypass path when formed permits a current flowing, in use, through
the DC path to flow the current bypass path and at the same time
bypass the current commutation device.
15. An electrical system comprising: a plurality of interconnected
DC transmission paths; and the electrical assembly according to
claim 1, one of the plurality of interconnected DC transmission
paths including the DC path of the electrical assembly, wherein the
control unit is programmed to selectively control the switching of
the switching element to commutate the current directly from the
switching element to the energy absorbing element in order to
increase the resultant voltage drop caused by the flow of direct
current through the DC path in which the current commutation device
is connected and thereby oppose the flow of the current through the
DC path to commutate the current from the DC path to the other DC
transmission path or at least one of the other DC transmission
paths.
16. The electrical system according to claim 15, wherein the
plurality of interconnected DC transmission paths are arranged so
that the voltage drop caused by, the flow of direct current through
the or each other of the plurality of interconnected DC
transmission paths is independent of the control of the current
commutation device to increase the resultant voltage drop caused by
the flow of direct current through the DC path.
17. The electrical system according to claim 16, wherein each one
of the plurality of interconnected DC transmission paths including
a respective one of the DC paths of the plurality of electrical
assemblies, wherein the plurality of interconnected DC transmission
paths are arranged so that the voltage drop caused by the flow of
direct current through each of the plurality of DC paths is
independent of the control of the respective current commutation
device to increase the resultant voltage drop caused by the flow of
direct current through the or each other of the plurality of DC
paths.
Description
BACKGROUND OF THE INVENTION
This invention relates to an electrical assembly and an electrical
system, in particular a high voltage direct current (HVDC) power
transmission network.
BRIEF DESCRIPTION OF THE INVENTION
An electrical system may include a power source, such as a battery,
that is connected to a load via one or more current-carrying
conductors, or multiple power sources that are connected to
multiple loads using a network of current-carrying conductors.
An example of an electrical system is a DC power grid that requires
multi-terminal interconnection of HVDC converters, whereby power
can be exchanged on the DC side using two or more HVDC converters
electrically connected together. Each HVDC converter acts as either
a source or sink to maintain the overall input-to-output power
balance of the DC power grid whilst exchanging the power as
required. The DC power grid relies on a network of DC power
transmission lines or cables to achieve multi-terminal
interconnection of the HVDC converters.
According to a first aspect of the invention, there is provided an
electrical assembly for use in an electrical system, the electrical
assembly comprising a DC path, the DC path including: a DC power
transmission medium, and a current commutation device, the current
commutation device including a switching element and an energy
absorbing element. The switching element is arranged to permit a
current flowing, in use, through the DC path to flow through the
switching element and at the same time bypass the energy absorbing
element. The electrical assembly further includes a control unit
programmed to selectively control the switching of the switching
element to commutate the current directly from the switching
element to the energy absorbing element in order to increase the
resultant voltage drop caused by the flow of direct current through
the DC path in which the current commutation device is connected
and thereby oppose the flow of the current through the DC path.
A DC power transmission medium may be any medium that is capable of
transmitting electrical power between two or more electrical
elements. Such a medium may be, but is not limited to, a submarine
DC power transmission cable, an overhead DC power transmission line
or cable and an underground DC power transmission cable.
The flow of electrical current through the DC path results in a
voltage drop that can change the flow of current in the DC power
transmission medium and associated DC electrical system. This
change in flow of current may result in the overloading of the DC
power transmission medium or any other DC power transmission medium
connected therewith, i.e. may cause the DC power transmission
medium or any other DC power transmission medium connected
therewith to operate beyond its rated conditions.
The provision of the current commutation device and control unit in
the electrical assembly enables the current flowing through the DC
path, and thereby through the switching element, to be directly
commutated to the energy absorbing element. Commutation of the
current flowing through the DC path from the switching element to
the energy absorbing element results in the increase of the
apparent resistance of the DC path that has the effect of opposing
the flow of the current in the DC path and thereby partly or wholly
redirecting (or diverting) the current into one or more other DC
transmission paths in the associated DC electrical system, thus
avoiding the undesirable effects caused by the overloading of the
DC power transmission medium.
It will be appreciated that the current commutation device and the
electrical assembly is not configured to be capable of breaking a
current flowing through the DC path, i.e. neither the current
commutation device nor the electrical assembly is a circuit
breaker. Accordingly there is no requirement for the current
commutation device or any of its components to have a high voltage
rating to enable it or the electrical assembly to break a current
flowing through the DC path.
Instead the current commutation device is configured to have a
voltage rating that enables the energy absorbing element to
increase the resultant voltage drop caused by the flow of direct
current through the DC path in which the current commutation device
is connected and thereby oppose the flow of the current through the
DC path.
For example, in a DC electrical system with an operating voltage
rating of 320 kV and a full load current rating of 1.5 kA, the DC
power transmission medium may have a length of 200 km, an operating
current rating of 1 kA and an operating voltage rating of 320 kV.
This results in the DC power transmission medium having a
resistance of approximately 2.4.OMEGA., which gives (at full rated
load for the DC power transmission medium) a voltage drop of
approximately 2.4 kV which is a dominant factor in determining the
voltage rating of the current commutation device of the invention.
Under such circumstances, an exemplary voltage rating of the
current commutation device may be 2 kV to 4 kV, which is roughly
two orders of magnitude smaller than the operating voltage rating
of the DC power transmission medium.
The configuration of the voltage rating of the current commutation
device in this manner not only permits the use of a relatively
small and low-cost current commutation device, but also results in
negligible losses in the current commutation device when compared
to the overall losses in the electrical system.
In the electrical assembly of the invention, the DC path may
further include: a mechanical switch connected to the DC power
transmission medium; and a controller configured to selectively
operate the mechanical switch to switch the DC power transmission
medium out of circuit. The control unit may be programmed to
selectively control the switching of the switching element to
commutate the current directly from the switching element to the
energy absorbing element to thereby force the current in the DC
power transmission medium to drop to a value that permits safe
opening of the mechanical switch prior to the controller operating
the mechanical switch to switch the DC power transmission medium
out of circuit.
The control unit may be programmed to selectively control the
switching of the switching element to commutate the current
directly from the switching element to the energy absorbing element
to thereby force the current in the DC power transmission medium to
drop to a value that permits arcless opening of the mechanical
switch prior to the controller operating the mechanical switch to
switch the DC power transmission medium out of circuit.
This results in an improved operation of the mechanical switch to
switch the DC power transmission medium out of circuit, since there
is no need for the formation of an arc in the mechanical switch.
This not only permits a reduction in the duty of the mechanical
switch, but also allows for a simpler design of the mechanical
switch.
An alternative to the invention would be to rely on the formation
of an arc in the mechanical switch, where the arc voltage provides
a back electromotive force (EMF) to oppose the flow of current
through the DC path. A second alternative to the invention would be
to connect each of a resonant circuit and a surge arrester in
parallel with the mechanical switch, where the formation of an arc
in the mechanical switch triggers a resonance in the resonant
circuit, and where a back EMF is generated by the surge arrester
after the arc is extinguished. In both alternatives, the mechanical
switch has to endure several milliseconds of arcing, with peak arc
currents reaching twice the value of the commutated current. Arcing
not only generates a substantial amount of heat, but also causes
pitting of the surface area of the contacts of the mechanical
switch. It can be, therefore, more difficult and expensive to
design a mechanical switch that needs to cope with arcing duty than
it is to design a mechanical switch that can be operated to open
under arcless conditions.
The control unit may be programmed to selectively control the
switching of the switching element to commutate the current
directly from the switching element to the energy absorbing element
to thereby damp any power oscillation present in the DC path prior
to the controller operating the mechanical switch to switch the DC
power transmission medium out of circuit.
This results in an improved operation of the mechanical switch to
switch the DC power transmission medium out of circuit, since the
current in the DC path can be indirectly reduced to damp any power
oscillation present in the DC path that would have otherwise
hampered the ability of the mechanical switch to safely open. This
can be particularly beneficial when it is difficult and/or
impractical to reduce the current to zero without disrupting the
flow of power in the associated DC electrical system, such as a
meshed DC electrical system.
Also, by configuring the current commutation device to enable
direct commutation of the current from the switching element to the
energy absorbing element, the electrical assembly is able to
respond quickly to a need to reduce the current in the DC path, for
example, in the event of a fault in the DC power transmission
medium. This in turn reduces the time delay in switching the DC
power transmission medium out of circuit.
The requirement to switch the DC power transmission medium out of
circuit may also arise under non-fault circumstances, which may
include operational circumstances such as DC power transmission
medium maintenance or segregation for transmission security
reasons.
The configuration of the electrical system in accordance with the
invention therefore enables the coordinated operations of the
current commutation device and mechanical switch to switch the DC
power transmission medium out of circuit whilst minimally impacting
the rest of the electrical system, thus permitting the rest of the
electrical system to continue normal service without
interruption.
One alternative to the invention would be to open DC circuit
breakers to interrupt the flow of current in a faulty DC power
transmission medium after the fault is detected and its location is
identified. Whilst the use of DC circuit breakers permits isolation
of the faulty DC power transmission medium to allow the rest of the
electrical system to continue its operation, presently available DC
circuit breakers tend to be relatively large, bulky and expensive
when compared to the electrical assembly of the invention.
Another alternative to the invention would be to block the flow of
power from one or more external sources into the electrical system
either by operating the associated converter(s) to block the flow
of power or by opening one or more AC circuit breakers connected
between the external source(s) and converter(s) if the associated
converter(s) is/are of the non-blocking type. This eventually
allows the current in the faulty DC power transmission medium to be
reduced to zero, and thereby allows the rest of the electrical
system to be restored to normal service. However, the current will
temporarily be in the form of energy "trapped" in the system
inductance of the DC electrical system and continues to persist
until the current decays through dissipation losses in the DC
electrical system. It typically takes several hundred milliseconds
for the current circulating in the DC electrical system to decay to
a value sufficiently low that would permit the restart of power
transmission. The combination of the time required for the decay in
the circulating current and the delays associated with opening and
reclosing the circuit breaker(s) connected between the external
source(s) and converter(s) could result in a significant period of
loss of transmission capability in the electrical system. This in
turn would have undesirable consequences on other electrical
systems and their components connected to the electrical
system.
A further alternative to the invention would be to configure the
electrical system to permit a shift from a symmetric voltage of
.+-.1 p.u. to an asymmetric voltage of 2 p.u. and 0 p.u., and to
rely on control action of the converters to reduce a current
flowing in a faulty DC power transmission medium to zero. Under
these conditions faults between the electrical system and ground
are considered to be high impedance faults and the current flowing
into the fault is relatively small. Once the location of the fault
is detected, control action of the converters can be then used to
force the current in the faulty DC power transmission medium to
zero, before mechanical switchgear, e.g. AC circuit breakers, is
operated to disconnect the faulty DC transmission path. Forcing the
current to zero in this manner, however, requires the coordination
of all the converters associated with the electrical system, thus
requiring a complex and expensive communications system to enable
performance of the coordination. In addition, it can be difficult
to implement such a communication system for certain topologies of
the electrical system. Furthermore, operation of the electrical
system at twice the nominal voltage for a significant period of
time would require all of its components and associated converters
to be suitably rated to handle twice the nominal voltage, thus
resulting in increased size, weight and costs of the electrical
system.
The configuration of the electrical system in accordance with the
invention provides a reliable means for reducing the flow of
current in a DC power transmission medium that minimises or
obviates the need for any of the aforementioned alternatives, thus
removing their associated disadvantages.
Furthermore, the configuration of the electrical system in
accordance with the invention does not require all of its
components and associated converters to be suitably rated to handle
twice the nominal voltage.
Moreover, the operation of the current commutation device to
directly commutate a current directly from the switching element to
the energy absorbing element may be optionally carried out
independently of the operation of the converters that form part of
or are associated with the electrical system. This is usually the
case when there are multiple alternative and/or parallel DC paths
between the converters in the DC electrical system, which may be in
the form of a meshed or grid system).
Alternatively, the operation of the current commutation device to
directly commutate a current directly from the switching element to
the energy absorbing element may be optionally carried with small
changes in the operation of the converters that form part of or are
associated with the electrical system. This is usually the case
when the DC path defines a single connection between a converter
(or a group of converters) and the rest of the DC electrical
system, e.g. a radial connection.
The mechanical switch may be incorporated into various DC switching
apparatus. For example, the DC path may include a DC switchgear,
the mechanical switch forming part of the DC switchgear, and/or
wherein the mechanical switch may be a disconnector (also known as
an isolator).
The configuration of the current commutation device may vary
depending on the requirements of the electrical system.
Optionally, in embodiments employing the use of the mechanical
switch, the current commutation device may be connected in series
with the mechanical switch. Such an arrangement provides a reliable
means of coordinating the operations of the current commutation
device and the mechanical switch to switch the DC power
transmission medium out of circuit.
In embodiments of the invention the switching element may be an
electronic switching element and/or a semiconductor switching
element. This further enhances the ability of the electrical
assembly to respond quickly to a need to control the current in the
DC path. The switching element may include a single switching
device or a plurality of switching devices, e.g. a plurality of
series-connected or parallel-connected switching devices.
In further embodiments of the invention the energy absorbing
element may include a linear resistor and/or a non-linear resistor.
The number of linear resistors and/or non-linear resistors in the
energy absorbing element may vary depending on the required control
of the current in the DC path, e.g. the required rate of change of
the current in the DC path.
The rating of the energy absorbing element is determined by the
rated direct current, the resistance of the DC power transmission
medium and the time required for the operation of the current
commutation device. For example, a rated direct current of 1.5 kA,
a resistance of the DC power transmission medium of 2.4.OMEGA. and
the time required for the operation of the current commutation
device of 100 ms would yield a rating of the energy absorbing
element of approximately 540 kJ. In practice, the rating of the
energy absorbing element may be less, but still in the range of a
few hundred kJ.
In still further embodiments of the invention the energy absorbing
element may be connected in parallel with the switching element.
Such an arrangement of the energy absorbing element and the
switching element provides a reliable means of directly commutating
the current from the switching element to the energy absorbing
element.
The components of the electrical assembly may vary in
arrangement.
In embodiments of the invention the current commutation device may
be connected in series with the DC power transmission medium. In
such embodiments employing the use of a mechanical switch, the
current commutation device may be used to both indirectly force the
current in the DC power transmission medium to drop to a value that
permits safe opening of the mechanical switch prior to the
controller operating the mechanical switch to switch the DC power
transmission medium out of circuit, and damp any power oscillation
present in the DC path prior to the controller operating the
mechanical switch to switch the DC power transmission medium out of
circuit.
In further embodiments of the invention the DC path may further
include an additional DC power transmission medium operably
connected to the DC power transmission medium.
In such embodiments employing the use of a mechanical switch, the
current commutation device may be connected in series with the
additional DC power transmission medium. In such embodiments the
current commutation device may be used to both indirectly force the
current in the additional DC power transmission medium to drop to a
value that permits safe opening of the mechanical switch prior to
the controller operating the mechanical switch to switch the
additional DC power transmission medium out of circuit, and damp
any power oscillation present in the DC path prior to the
controller operating the mechanical switch to switch the additional
DC power transmission medium out of circuit.
In other such embodiments the DC path may further include an
additional current commutation device, the current commutation
device and the additional current commutation devices being
connected in series with the DC power transmission medium and the
additional DC power transmission medium respectively.
In such embodiments employing the use of a mechanical switch, the
current commutation device may be used to indirectly force the
current in the DC power transmission medium to drop to a value that
permits safe opening of the mechanical switch prior to the
controller operating the mechanical switch to switch the DC power
transmission medium out of circuit, and the additional current
commutation device may be used to damp any power oscillation
present in the DC path prior to the controller operating the
mechanical switch to switch the DC power transmission medium out of
circuit.
In other such embodiments the additional current commutation device
may be used to indirectly force the current in the DC power
transmission medium to drop to a value that permits safe opening of
the mechanical switch prior to the controller operating the
mechanical switch to switch the DC power transmission medium out of
circuit, and the current commutation device may be used to damp any
power oscillation present in the DC path prior to the controller
operating the mechanical switch to switch the DC power transmission
medium out of circuit.
Accordingly, the current commutation device and the additional
current commutation device can be optimised independently of each
other and in accordance with their respective current control
duties.
In embodiments of the invention employing the use of a mechanical
switch, both current commutation devices may be used to force the
current in the DC power transmission medium to drop to a value that
permits safe opening of the mechanical switch prior to the
controller operating the mechanical switch to switch the DC power
transmission medium out of circuit, and/or both current commutation
devices may be used to damp any power oscillation present in the DC
path prior to the controller operating the mechanical switch to
switch the DC power transmission medium out of circuit.
In embodiments of the invention the control unit may be programmed
to selectively switch the switching element on and off a plurality
of times to control the commutation of the current directly from
the switching element to the energy absorbing element. Repeatedly
switching the switching element on and off results in the
controlled variation of the apparent resistance of the DC path.
Such variation can be performed to modify the flow of the current
in the DC path. For example, in view of the time-varying nature of
power oscillations, the switching element may be switched on and
off a plurality of times to damp one or more power oscillations
present in the DC path.
Optionally, the DC path may further include a current bypass device
arranged to permit selective formation of a current bypass path,
and the current bypass path when formed permits a current flowing,
in use, through the DC path to flow the current bypass path and at
the same time bypass the current commutation device. This permits
the current commutation device to be bypassed under high fault
current conditions or when the current commutation device is
faulty.
According to a second aspect of the invention, there is provided an
electrical system comprising a plurality of interconnected DC
transmission paths, and an electrical assembly, one of the
plurality of interconnected DC transmission paths including the DC
path of the electrical assembly. The control unit is programmed to
selectively control the switching of the switching element to
commutate the current directly from the switching element to the
energy absorbing element in order to increase the resultant voltage
drop caused by the flow of direct current through the DC path in
which the current commutation device is connected and thereby
oppose the flow of the current through the DC path to commutate the
current from the DC path to the other DC transmission path or at
least one of the other DC transmission paths.
The plurality of interconnected DC transmission paths may be
arranged so that the voltage drop caused by the flow of direct
current through the or each other of the plurality of
interconnected DC transmission paths is independent of the control
of the current commutation device to increase the resultant voltage
drop caused by the flow of direct current through the DC path.
Such an electrical system may be, for example, a multi-terminal DC
electrical network including a plurality of DC terminals, whereby
each DC transmission path is connected between multiple DC
terminals, or may be an HVDC station.
The electrical system of the invention may include a plurality of
electrical assemblies according to any one of the embodiments of
the first aspect of the invention, wherein each one of the
plurality of interconnected DC transmission paths including a
respective one of the DC paths of the plurality of electrical
assemblies. The plurality of interconnected DC transmission paths
are arranged so that the voltage drop caused by the flow of direct
current through each of the plurality of DC paths is independent of
the control of the respective current commutation device to
increase the resultant voltage drop caused by the flow of direct
current through the or each other of the plurality of DC paths.
In such embodiments multiple current commutation devices may be
simultaneously operated to control the redistribution of the
current between the various DC paths.
The corresponding advantages described above with reference to the
first aspect of the invention apply mutatis mutandis to the second
aspect of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiments of the invention will now be described, by
way of non-limiting examples, with reference to the accompanying
drawings in which:
FIG. 1 shows schematically an electrical system in the form of a DC
power grid according to a first embodiment of the invention;
FIG. 2 shows schematically an electrical assembly according to a
second embodiment of the invention that forms part of the DC power
grid of FIG. 1; and
FIG. 3 shows schematically an electrical system in the form of a
HVDC station according to a third embodiment of the invention.
DETAILED DESCRIPTION OF THE DRAWINGS
An electrical system in the form of a DC power grid according to a
first embodiment of the invention is shown in FIG. 1 and is
designated generally by the reference numeral 200.
The DC power grid 200 comprises a plurality of DC terminals 202,
and a plurality of DC power transmission lines 204A, 204B, 204C. In
use, each DC terminal 202 is operatively connected to the DC side
of a respective AC-DC converter 206, which in turn is connected to
a respective AC electrical network 208, 209.
In the embodiment shown in FIG. 1, a first DC power transmission
line 204A is arranged to interconnect a first DC terminal 202 and a
second DC terminal 202, a second DC power transmission line 204B is
arranged to interconnect the first DC terminal 202 and a third DC
terminal (not shown), and a third DC power transmission line 204C
is arranged to interconnect the second DC terminal 202 and a fourth
DC terminal (not shown).
Each end of each DC power transmission line 204A, 204B, 204C is
connected in series with a respective disconnector 210. In use,
each disconnector 210 can be operated to open to selectively block
current from flowing in the corresponding DC power transmission
line 204A, 204B, 204C when the current flowing in the corresponding
DC power transmission line 204A, 204B, 204C is at a current
threshold of zero or substantially zero, i.e. a value that permits
safe opening of each disconnector 210.
Each series-connection of each DC power transmission line 204A,
204B, 204C and the corresponding disconnectors 210 is further
connected in series with a respective current commutation device
212 to define a respective electrical assembly.
As shown in FIG. 2, each current commutation device 212 includes a
switching element 214 connected in parallel with an energy
absorbing element 216. The switching element 214 includes a pair of
inverse-series connected switching devices in the form of insulated
gate bipolar transistors (IGBT), whereby each IGBT is connected in
parallel with an anti-parallel diode. The energy absorbing element
216 includes a surge arrester, e.g. a zinc-oxide surge
arrester.
It is envisaged that, in other embodiments of the invention, the
number of switching devices in each switching element 214 may vary
and/or the number of energy absorbing elements 216 in each current
commutation device 212 may vary. It is further envisaged that, in
still other embodiments of the invention, the switching element 214
may instead or additionally include different switching devices,
such as IGBTs, IGCTs, GTO, other self-commutating switching
devices, etc and/or the energy absorbing element 216 may instead or
additionally include another type of non-linear resistor and/or a
linear resistor.
Each electrical assembly further includes a respective controller
218 configured to selectively operate the corresponding
disconnector 210 to disconnect the corresponding DC power
transmission line 204. Each controller 218 may be programmable to
operate the corresponding disconnector 210 or may be configured to
mechanically operate the corresponding disconnector 210.
Each electrical assembly further includes a control unit 220
programmed to selectively control the switching of the switching
element 214, i.e. to selectively turn on and off the switching
element 214.
It will be appreciated that some or all of the controllers 218 and
control units 220 may be separate from each other. It will be
further appreciated that some or all of the controllers 218 and the
control units may be integral with each other 220.
During normal operation, the DC power grid 200 is used to transfer
power, for example, from offshore wind farms 208 to on-shore AC
systems 209 via the converters 206 and the DC power transmission
lines 204A, 204B, 204C. During such normal operation, a current
flows through each DC power transmission line 204A, 204B, 204C, and
therefore also flows through each disconnector 210 and each current
commutation device 212. More specifically, when the current flows
through a given current commutation device 212, the parallel
connection of the switching element 214 and the energy absorbing
element 216 means that the current preferentially flows through the
switching element 214 due to the resistance of the energy absorbing
element 216 when the switching element 214 is switched on.
A fault or other abnormal operating condition may occur in the DC
power grid 200. For example, one of the DC power transmission lines
204A, 204B, 204C of the DC power grid 200 may experience a
pole-to-ground fault. The presence of the fault or other abnormal
operating condition may result in a high fault current in the
faulty DC power transmission line 204A, 204B, 204C and therefore in
the DC power grid 200.
For the purposes of illustrating how the invention works, it is
assumed that the first DC power transmission line 204A is the
faulty DC power transmission line, but it will be understood that
the following description of the working of the invention applies
mutatis mutandis to a fault occurring in any of the other DC power
transmission lines 204B, 204C.
In a first example of the working of the invention, the first DC
power transmission line 204A, the corresponding disconnectors 210
and the corresponding current commutation device 212 together
define a DC path.
Following occurrence of the fault in the first DC power
transmission line 204A, the control unit 220 controls switching of
the switching element 214 to open and thereby commutate the current
directly from the switching element 214 to the energy absorbing
element 216. The commutation of the current flowing through the DC
path directly from the switching element 214 to the energy
absorbing element 216 results in the increase of the resultant
voltage drop caused by the flow of direct current through the DC
path and therefore the apparent resistance of the DC path that has
the effect of opposing the flow of the current in the DC path and
thereby causing it to wholly or partly flow elsewhere in the DC
power grid 200, i.e. wholly or partly redirect the current into the
other DC power transmission lines 204B, 204C. In this instance the
resistance of the energy absorbing element 216 is suitably rated to
indirectly force the current in the first DC power transmission
line 204A to drop to a value that permits safe opening of the
disconnectors 210 prior to the controllers 218 operating the
disconnectors 210 to disconnect the first DC power transmission
line 204A. In this case the target value is zero or substantially
zero.
Meanwhile the fault in the first DC power transmission line 204A
may result in the occurrence of power oscillations in the DC path.
Additionally or alternatively the power oscillations in the DC path
could be caused by the converters 206 or the AC electrical networks
208, 209, or by one or more faults occurring elsewhere in the DC
power grid 200. Thus, a direct current plus one or more oscillatory
components will flow in the DC path as a result of the inductance
and capacitance of the DC path. Whilst the power oscillations will
naturally dampen with time, such damping could take a considerable
amount of time so as to hamper the ability of the disconnectors 210
to safely open to disconnect the first DC power transmission line
204.
In view of the time-varying nature of the power oscillations, the
control unit may optionally switch the switching element 214 on and
off a plurality of times at a specific frequency (which may range
from a few Hz to a few kHz) to control the commutation of current
directly from the switching element 214 to the energy absorbing
element 216 to damp the power oscillations present in the DC path
prior to the controllers 218 operating the disconnectors 210 to
disconnect the first DC power transmission line 204A. Repeatedly
switching the switching element 214 on and off at the specific
frequency results in the controlled variation of the apparent
resistance of the DC path, whereby the controlled apparent
resistance can be varied to modify the flow of the current in the
DC path.
The switching of the switching element 214 on and off a plurality
of times may be carried out as a pulse width modulation at a
frequency of, for example, 500 Hz.
When the fault current in the first DC power transmission line 204A
is reduced to the target value of zero or substantially zero, the
controllers 218 operate the disconnectors 210 connected at both
ends of the first DC power transmission line 204A to open and
thereby block current from flowing in the faulty first DC power
transmission line 204A.
Meanwhile the rest of the DC power grid 200 is able to continue its
normal service without interruption.
By configuring the current commutation device 212 to enable direct
commutation of the current from the switching element 214 to the
energy absorbing element 216, the electrical assembly is able to
respond quickly to a need to control the current in the DC path.
This in turn reduces the time delay in disconnecting the first DC
power transmission line 204A.
The inclusion of the switching element 214 in the electrical
assembly not only permits the use of a relatively small and
low-cost switching element 214, but also results in negligible
losses in the current commutation device 212 when compared to the
overall losses in the DC power grid 200.
In a second example of the working of the invention, the
disconnectors 210 connected at both ends of the first DC power
transmission line 204A may be permitted to safely open through
operation of the current commutation device 212 corresponding to
another DC power transmission line 204B, 204C, which in the
embodiment shown may be the second or third DC power transmission
line 204B, 204C. In this example, the first DC power transmission
line 204A, the other DC power transmission line 204B, 204C, the
corresponding disconnectors 210 and the corresponding current
commutation devices 212 together define a DC path.
The working of the invention in the second example is identical to
the working of the invention in the first example, except that the
operation of the current commutation device 212 corresponding to
the first DC power transmission line 204A is replaced by the
operation of the current commutation device 212 corresponding to
the other DC power transmission line 204B, 204C. Whilst this still
results in the increase of the resultant voltage drop caused by the
flow of direct current through the DC path and therefore the
apparent resistance of the DC path that has the effect of opposing
the flow of the current in the DC path and thereby cause it to flow
elsewhere in the DC power grid 200, the resistance of the energy
absorbing element 216 is suitably rated to indirectly (as opposed
to directly) force the current in the first DC power transmission
line 204A to drop to a value that permits safe opening of the
disconnectors 210 corresponding to the first DC power transmission
line 204A prior to the controllers 218 operating the disconnectors
210 to disconnect the first DC power transmission line 204A. The
current commutation device 212 corresponding to the other DC power
transmission line 204B, 204C can be operated to increase the
resultant voltage drop caused by the flow of direct current through
the DC path and therefore the apparent resistance of the DC path in
order to damp the power oscillations present in the DC path prior
to the controllers 218 operating the disconnectors 210 to
disconnect the first DC power transmission line 204A.
In a third example of the working of the invention, the
disconnectors 210 connected at both ends of the first DC power
transmission line 204A may be permitted to safely open through
operation of both of the current commutation devices 212
corresponding to the first DC power transmission line 204A and the
other DC power transmission line 204B, 204C. In this example, the
first DC power transmission line 204A, the other DC power
transmission line 204B, 204C, the corresponding disconnectors 210
and the corresponding current commutation devices 212 together
define a DC path.
In this example, the operation of the current commutation device
212 corresponding to the first DC power transmission line 204A is
complemented by the operation of the current commutation device 212
corresponding to the other DC power transmission line 204B,
204C.
The current commutation device 212 corresponding to the first DC
power transmission line 204A may be operated to indirectly force
the current in the first DC power transmission line 204A to drop to
a value that permits safe opening of the disconnectors 210
corresponding to the first DC power transmission line 204A prior to
the controllers 218 operating the disconnectors 210 to disconnect
the first DC power transmission line 204A, while the current
commutation device 212 corresponding to the other DC power
transmission line 204B, 204C may be operated to increase the
resultant voltage drop caused by the flow of direct current through
the DC path and therefore the apparent resistance of the DC path in
order to damp the power oscillations present in the DC path prior
to the controllers 218 operating the disconnectors 210 to
disconnect the first DC power transmission line 204A.
Alternatively the current commutation device 212 corresponding to
the other DC power transmission line 204B, 204C may be operated to
indirectly force the current in the first DC power transmission
line 204A to drop to a value that permits safe opening of the
disconnectors 210 corresponding to the first DC power transmission
line 204A prior to the controllers 218 operating the disconnectors
210 to disconnect the first DC power transmission line 204A, while
the current commutation device 212 corresponding to the first DC
power transmission line 204A may be operated to increase the
resultant voltage drop caused by the flow of direct current through
the DC path and therefore the apparent resistance of the DC path in
order to damp the power oscillations present in the DC path prior
to the controllers 218 operating the disconnectors 210 to
disconnect the first DC power transmission line 204.
Further alternatively both current commutation devices 212 may be
operated to force the current in the first DC power transmission
line 204A to drop to a value that permits safe opening of the
disconnectors 210 corresponding to the first DC power transmission
line 204A prior to the controllers 218 operating the disconnectors
210 to disconnect the first DC power transmission line 204A, and/or
both current commutation devices 212 may be operated to increase
the resultant voltage drop caused by the flow of direct current
through the DC path and therefore the apparent resistance of the DC
path in order to damp the power oscillations present in the DC path
prior to the controllers 218 operating the disconnectors 210 to
disconnect the first DC power transmission line 204A.
The requirement to disconnect a given DC power transmission line
204A, 204B, 204C may also arise under non-fault circumstances,
which may include operational circumstances such as DC power
transmission line maintenance or segregation for transmission
security reasons.
An electrical system in the form of a HVDC station according to a
third embodiment of the invention is shown in FIG. 3 and is
designated generally by the reference numeral 300.
FIG. 3 shows a single-line diagram of the HVDC station, which
comprises a first pole 302, a second pole 304, an electrode line
306, and switching valves 308 connected between the first pole 302
and the electrode line 306.
The HVDC station 300 includes a plurality of DC paths, each of
which includes a respective DC switchgear 310 connected to a DC
power transmission medium. The plurality of DC switchgears 310
includes: a plurality of bypass switches 312, each of which is
connected in parallel with a respective one of the switching valves
308; a neutral bus switch 314 connected between the switching
valves 308 and the electrode line 306; a neutral bus grounding
switch 316 connected between the electrode line 306 and ground; a
metallic return transfer switch 318 connected in the electrode line
306; and a ground return transfer switch 320 connected between the
electrode line 306 and a mid-point between the first and second
poles 302, 304.
Each DC switchgear 310 is identical in structure to the arrangement
shown in FIG. 2 in that each DC switchgear 310 includes a
mechanical switch connected in series with a current commutation
device 212, where the mechanical switch is in the form of a
disconnector 210. It is envisaged that, in other embodiments of the
invention, the disconnector 210 may be replaced by another type of
mechanical switch.
In use, each DC switchgear 310 can be operated to open to
selectively block current from flowing in the corresponding DC
power transmission medium through the operation of the
corresponding disconnector 210 to disconnect the corresponding DC
power transmission medium, when the current flowing in the
corresponding DC power transmission medium is at a current
threshold of zero or substantially zero, i.e. a value that permits
arcless opening of each disconnector 210.
The connection of each DC switchgear 310 and the corresponding DC
power transmission medium defines a respective electrical
assembly.
Each electrical assembly further includes a respective controller
218 configured to selectively operate the corresponding
disconnector 210 to disconnect the corresponding DC power
transmission medium. Each controller 218 is configured to
mechanically operate the corresponding disconnector 210. Each
electrical assembly further includes a control unit 220 programmed
to selectively control the switching of the switching element 214,
i.e. to selectively turn on and off the switching element 214.
During the operation of the HVDC station 300, direct current flows
through each DC path when the corresponding DC switchgear 310 is
closed. The current flows through the closed DC switchgear 310 and
corresponding DC power transmission medium, and therefore also
flows through the corresponding disconnector 210 and current
commutation device 212. Similarly to the first embodiment of the
invention, when the current flows through a given current
commutation device 212, the parallel connection of the switching
element 214 and the energy absorbing element 216 means that the
current preferentially flows through the switching element 214 due
to the resistance of the energy absorbing element 216 when the
switching element 214 is switched on.
It may be required to open a given closed DC switchgear 310 to
commutate a direct current as part of its normal duty, which could
arise under fault or non-fault circumstances.
The opening of the given closed DC switchgear 310 to commutate a
direct current is described as follows.
Initially the control unit 220 controls switching of the switching
element 214 to open and thereby commutate the current directly from
the switching element 214 to the energy absorbing element 216. The
commutation of the current flowing through the DC path directly
from the switching element 214 to the energy absorbing element 216
results in the increase of the resultant voltage drop caused by the
flow of direct current through the DC path and therefore the
apparent resistance of the DC path that has the effect of opposing
the flow of the current in the DC path and thereby causing it to
wholly or partly flow elsewhere in the HVDC station 300. In this
instance the resistance of the energy absorbing element 216 is
suitably rated to indirectly force the current in the corresponding
DC power transmission medium to drop to a value that permits
arcless opening of the corresponding disconnector 210 prior to the
controller 218 operating the disconnector 210 to disconnect the
corresponding DC power transmission medium. In this case the target
value is zero or substantially zero.
When the current in the corresponding DC power transmission medium
is reduced to the target value of zero or substantially zero, the
controller 218 operates the disconnector 210 to open under arcless
conditions and thereby block current from flowing in the
corresponding DC power transmission medium, thus effectively
disconnecting the corresponding DC power transmission medium. The
opening of the disconnector 210 also provides high insulation
between the terminals of the DC switchgear 310.
Since each DC switchgear 310 is not required to operate as a
circuit breaker to break the direct current flowing therethrough
but only required to commutate the direct current from the
corresponding DC path to elsewhere in the HVDC station 300, the
increase in the resultant voltage drop due to the commutation of
the current directly from the switching element 214 to the energy
absorbing element 216 is relatively low when compared to the
nominal voltage rating of the HVDC station 300. The voltage rating
of the switching element 214 can be configured to be relatively
small and can be achieved with a low number of switching devices or
a single bidirectional switching device.
The inclusion of the current commutation device 212 in each
electrical assembly not only permits arcless opening of the
mechanical switch of each DC switchgear 310, but also permits the
use of a relatively small and low-cost switching element 214.
Furthermore, the energy absorbing element 216 protects the
switching element 214 from voltage spikes, which may arise during
the operation of the DC switchgear 310.
It will be appreciated that the control of a given current
commutation device 212 to commutate the current flowing through the
corresponding DC path directly from the switching element 214 to
the energy absorbing element 216 has the effect of opposing the
flow of the current only in the DC path in which the given current
commutation device 212 is connected. In other words, the plurality
of DC paths are arranged so that the voltage drop caused by the
flow of direct current through each of the plurality of DC paths is
independent of the control of the respective current commutation
device 212 to increase the resultant voltage drop caused by the
flow of direct current through each other of the plurality of DC
paths.
It is envisaged that, in other embodiments, each DC power
transmission line 204A, 204B, 204C may be replaced by, but is not
limited to, a submarine DC power transmission cable, an overhead DC
power transmission cable, an underground DC power transmission
cable, or any DC power transmission medium of transmitting
electrical power between two or more electrical elements.
It will be appreciated that the topologies and configurations of
the electrical system, the electrical assembly and the current
commutation device 212 were merely chosen to illustrate the working
of the invention and that the invention is applicable to other
topologies and configurations of the electrical system, the
electrical assembly and the current commutation device.
* * * * *